#### Answer

($r^{3}$ - 16)($r^{3}$ - 5)

#### Work Step by Step

We write the polynomial
$(r^{3})^{2}$ - 21$r^{3}$ + 80
*** We break of the middle term into two factors that add to give -21 and multiply to give +80. The two numbers are -16 and -5.
$(r^{3})^{2}$ - 16$r^{3}$ - 5$r^{3}$ + 80
We take the GCD of the first two and the GCD of the last two terms.
$r^{3}$($r^{3}$ - 16) -5 ($r^{3}$ - 16)
We take ($r^{3}$ - 16) and factor it out which gives us.
($r^{3}$ - 16)($r^{3}$ - 5)